Modern cars comprise electronic control systems as anti-lock-braking systems (ABS), dynamic stability systems, anti-spin systems and traction control systems. Besides these active control systems there also exist driver safety information systems as road friction indicators and tire pressure monitoring systems which present information about the driving condition to the driver. Some types of these tire pressure monitoring systems determine lowering of the tire pressure based on indirect detection values of a modern vehicle, such as the wheel speed signals, etc.
Known indirect tire pressure monitoring systems are based on either monitoring a change in roll-radius or vibration spectra derived from the wheel speed signals. Such a roll-radius approach is disclosed in the Japanese application JP 5-55322, where wheel rolling speeds derived from the tire's specific wheel speed signals are compared to each other for each pair of diagonally disposed tires. Such systems can detect one or three under-inflated tires. Model-based roll-radius approaches using longitudinal and lateral dynamic models can detect one, two or three tires with low pressure. However, roll-radius approaches cannot solve the problem with diffusion, when all four wheels slowly become under-inflated. Such a situation may be detected with the known vibration spectrum approach which is based on the physical fact that the spring-damper dynamics in the tire changes with air pressure. The Japanese patent JP 2836652 discloses a system adopting this approach for detecting under-inflated tires, based on the property that the resonance peak in the vibration spectrum shifts in frequency after a pressure change, and this peak frequency value is continuously monitored and compared to a reference value. This patent also discloses to adapt this reference value with the vehicle's speed. The patent EP 0 783 982 B1 adopting the same approach further includes the case of monitoring several resonance peaks from the upward-downward and forward-backward movements of an unsprung mass (wheel). The main drawbacks of monitoring only the peak value is a high sensitivity to the tire type, vehicle speed and road surface. Similarly, EP 578 826 A1 discloses to detect resonance peaks related to vertical and longitudinal directions of the vehicle's unsprung mass and to derive the tire pressure from the resonance frequencies of these resonance peaks.
The patent US 2004/0260436 monitors the whole spectrum, rather than just the peak value, by cross correlating the current spectrum with a reference spectrum in order to decrease the sensitivity to the tire type, vehicle speed and road surface. One drawback with this method is the large requirement of computational speed and memory to compute the spectrum at a large number of frequency grid points. Another drawback is the lack of robustness to many problems encountered in practice, such as spurious frequency components in the spectrum either caused by random noise or by interference from other rotating parts of the vehicle and its driveline. A random noise component in the current spectrum can give a large difference in the computed correlation value between reference and current spectrum, and thus may cause false alarms or missed detections.
The general problem to be solved by the present invention is to improve the performance of an indirect tire pressure monitoring system.
The problem will be solved by the subject matter of the independent claims. Preferred embodiments are disclosed in the dependent claims.
A first aspect of the invention is directed to a method of estimating a tire pressure deviation of a vehicle's tire. The method comprises the steps of receiving as an input signal a vehicle signal, calculating at least one shape factor of a is resonance peak associated with the vehicle signal spectrum on the basis of the input signal; and determining, depending on said at least one shape factor, a tire pressure signal indicative of a tire pressure deviation within the vehicle's tire.
Another aspect of the invention is directed to a system of estimating the tire pressure deviation. The system comprises a receiving section adapted to receive as an input signal a vehicle signal, a calculation section adapted to calculate at least one shape factor of a resonance peak associated with the vehicle signal spectrum on the basis of the input signal, and a determining section adapted to derive, depending on said at least one shape factor, a tire pressure signal indicative of a tire pressure deviation within the vehicle's tire.
A further aspect of the invention is directed to a computer program including program code for carrying out a method, when executed on a processing system, of estimating a tire pressure deviation of a driving vehicle, the method comprises the steps of receiving as an input signal a vehicle signal, calculating at least one shape factor of a resonance peak associated with the vehicle signal spectrum on the basis of the input signal, and determining, depending on said at least one shape factor, a tire pressure signal indicative of a tire pressure deviation within the vehicle's tire.